Minor refactor

Just move when we cache the accessor for preprocess in Airport.
Rework computing the intersect point

We now use a better formula provided by Dr. Zirbel in computing the end points
of the interaction lines. It is not yet complete as one factor still needs to
be scaled with font size, however it works nicely now. This is much cleaner
than my previous code.
Clean up some commented out code
Large refactor of drawing and highlighting

Now nucleotides are highlighted in a consitent way across views. This has
promoted the addition of a highlights component. In addition, the way things
are drawn is changed. Now each view defines a callback that handles only adding
the data it must for nts, chains, interactions, etc. All the standard parts are
defined in the parent view class.